The precise measurement of the time of arrival (TOA) of an RF pulse envelope is of substantial importance to radar equipment. An accurate TOA makes it possible for a radar, such as a radio directional finder or emitter (transmitter) locater, to determine the direction from which a signal is received, thus, revealing the direction of the receiver with respect to the transmitting station and vice-versa, as well as determining the location of the transmitting station. Since the precise TOA measurements are in terms of absolute time, it is possible to relate the time measurements from two different receivers, separated by a substantial distance, and accomplish bistatic transmitter station location. Normally, as part of a TOA measurement, a precise pulse width measurement is also accomplished which allows for the identification of a particular transmitter, within a group of similar transmitters, by detecting minor differences in the particular circuits which transmit the RF signal having an identifiable pulse width.
Earlier techniques for precision time measurement have included a so called "snapshot digitizer," which digitizes the RF pulse envelope at a very high rate, of the order of 100 MHz. In such a circuit, following the detection of the presence of the RF pulse envelope, digitizing continues for a predetermined duration and, then, the digital samples are transferred to a computer wherein the samples are analyzed to determine the TOA and pulse width of the RF pulse envelope. This is usually a relatively slow and expensive process and, further, even with the 100 MHz sampling rate, the snapshot digitizer requires interpolation between the samples separated by 10 nanoseconds to achieve a resolution in the order of 1 nanosecond. It is desired that a less expensive yet more accurate time measurement device be provided.
A second known technique for measuring TOA and pulse width is to employ a very fast clock, e.g., 1,000 MHz, and a detection circuit, such as a threshold detector, to detect the time when the arriving RF pulse envelope reaches a predetermined threshold value. The accuracy of this technique is dependent upon, among other things, both the rise time and signal strength of the received RF pulse envelope. For example, if the received RF envelope has a 100 nanosecond rise time, from the predetermined threshold point crossing to the half-power point of the RF pulse envelope, then this technique would exhibit a 100 nanosecond variation in its measurement when the signal strength is reduced until the half-power point of the RF pulse envelope is exactly at the predetermined threshold value.
A phase interferometer, having closely spaced and matched receivers, is sometimes used to provide a precision type measurement of received RF signals. The received RF signals; i.e., an RF pulse envelope comprising an RF carrier, are split into two separate paths and the phase difference is measured therebetween. This device is somewhat limited in that the accuracy of its time measurement is dependent upon the closely-spaced and matched signal receivers.
It is desired that a circuit arrangement and a method of use thereof be provided that would overcome the drawbacks of the prior art which appear to be limited to devices requiring matched receivers, or relatively expensive but slow measurement techniques, all of which have an accuracy that is limited to about 1 nanosecond.
It is, therefore, a primary object of the present invention to provide a circuit arrangement for measuring the RF pulse amplitude to accurately determine the time of arrival (TOA) and pulse width thereof, within an accuracy of less than 1 nanosecond.
It is another object of the present invention to provide a method for measuring the RF pulse amplitude to determine, within an accuracy of less than 1 nanosecond, its time of arrival (TOA) and pulse width.
It is a further object of the present invention to provide accurate time of arrival (TOA) and pulse width measurements of an RF pulse envelope at predetermined and selectable pulse width points, such as its 3 dB points.
Other objects of the present invention, as well as advantages thereof over existing and prior art forms, which will be apparent in view of the following detailed description, are accomplished by means hereinafter described and claimed.